1. Field of the Invention
The present invention relates to a method for manufacturing a disc-shaped optical recording medium and a disc-shaped substrate processed thereto.
2. Description of the Related Art
Disc-shaped optical recording media (optical discs) such as CD (Compact Disc) and DVD (Digital Versatile Disc) have an injection-molded disc-shaped substrate including an information recording surface. In a predetermined region of the information recording surface of the substrate, a writable recording layer for recording information, a reflective layer to allow reproduction, and the like are formed. Then, a resinous protection layer is formed thereon to complete the disc-shaped optical recording media. When a prescribed laser beam is irradiated through the disc-shaped substrate, information is recorded on and/or reproduced from the media.
Recently, another optical disc has been gaining attention. The disc is produced by a method, as disclosed in Japanese Patent Laid-Open Publication No. Hei. 8-235638, for example, in which a support layer (protection layer) that does not have to transmit light, in other words, without optical thickness requirement is formed to be thick by injection molding as a disc-shaped substrate. A reflective layer to allow reproduction, a writable recording layer and the like are formed on the information recording surface side of the substrate, and then a light transmitting layer of transparent resin that can transmit a laser beam for recording/reproducing is formed on the top of the other layers.
According to a conventional method of manufacturing an optical disc, when the substrate is injection molded, a center portion of the substrate and a sprue runner-solidified inside a gate for use in injection molding are removed to form a circular central hole in the center of the substrate. Then, a reflective layer and a writable recording layer, as described above, are formed on the substrate. A resinous protection layer and a-light transmitting layer of light transmitting resin are then formed by spin-coating, for example, in the next process, and then the optical disc is completed.
As disclosed by Japanese Patent Laid-Open Publication No. Hei. 10-249264, one of characteristic manufacturing methods of such a protection layer or a light transmitting layer is that a lid member is fitted into the central hole of the optical disc to cover the hole, over which resin is spread over the whole surface of the optical disc by spin-coating, and cured to form the protection layer (resin layer). This method can easily control thickness of the resin layer, so that it is possible to form the resin layer to have uniform thickness, particularly, in a radial direction. Thus, the method is applicable to the formation of the light transmitting layer as disclosed by Japanese Patent Laid-Open Publication No. Hei. 8-235638.
As described above, the method with the lid member for covering the central hole of the substrate is effective to form a more uniform resin layer in spin-coating of the resin layer. However, there is a disadvantage that the method has to always use a new (clean) lid member. Thus, a cleaning process for cleaning the lid member is required each time after the spin-coating of resin, or the lid member has to be disposable.
Therefore, the present inventors have invented a manufacturing method of a substrate according to which a central hole for optical disc chucking is formed after the resin layer is formed based on a different point of view from the above-mentioned conventional art. According to this manufacturing method, it is possible to simplify manufacturing processes and to improve a yield in forming of the resin layer as compared to the conventional method using a lid member. They have also suggested a disc-shaped optical recording medium which has no central hole and in which the recording layer extends up to the region corresponding to the central hole and the region for chucking in the prior art disc.
During a sputtering process for forming the recording layer, etc, the substrate has to be fixed in a predetermined position, and the central hole of the conventional substrate formed at the time of injection molding is used as a chuck for fixing its position. Thus, the substrate without any central hole cannot be positioned by the conventional fixing method.
In this case, it is conceivable to use an outer mask, which covers an outer periphery of the substrate during the sputtering, to fix the position of the substrate.
However, it is difficult to surely position and hold the substrate during the sputtering only with the outer mask. The substrate is also easily displaced due to the deformation of the substrate caused by the heat generated during the sputtering.
The present invention is directed to a solution to the above described disadvantage, and it is an object of the present invention to provide a method for manufacturing a disc-shaped optical recording medium according to which a substrate can surely be positioned and held at the time of sputtering. According to the method, a layer is formed on the substrate without a central hole for optical disk chucking by sputtering before a central hole may be formed. It is another object of the invention to provide a disc-shaped substrate for optical recording medium.
After intensive research, the inventors have found that the above-described disadvantage is solved by the use of a positioning guide, for example, having circular recesses on a non-sputtering surface of a disc-shaped substrate in positioning during the sputtering process.
That is, the present invention described below allows the aforementioned objects to be achieved.
(1) A method for manufacturing a disc-shaped optical recording medium comprising: a step of forming a disc-shaped substrate without a central hole for optical medium chucking, so that said disc-shaped substrate has a positioning guide formed in a non-sputtering surface being one surface of said disc-shaped substrate; at least one sputtering step of forming a film on a sputtering surface with sputtering of said disc-shaped substrate positioned by a carrier, while said positioning guide positions said disc-shaped substrate in place.
(2) The method according to (1), wherein said positioning guide is a circular recess formed at a center of said disc-shaped substrate during the step of forming the disc-shaped substrate without a central hole, said circular recess having a depth not large enough to penetrate through the disc-shaped substrate in a thickness-wise direction.
(3) The method according to (2), wherein said carrier is provided with an outer mask for covering an outer periphery of said disc-shaped substrate and a positioning member engaged with said circular recess, and said outer mask and said positioning member fixes a position of said disc-shaped substrate when the sputtering is carried out.
(4) The method according to (3), wherein said positioning member protrudes in a direction of pushing said disc-shaped substrate to detach said disc-shaped substrate from said carrier by pushing the substrate with said positioning member from its backside after the sputtering.
(5) The method according to claim 1, wherein said disc-shaped substrate is pushed out from its backside after the sputtering.
(6) The method according to any one of (2), (3) and (4), further comprising the step of perforating a region including said circular recess of said disc-shaped substrate after the sputtering step to form a central hole for optical medium chucking.
(7) The method according to (1) or (2), wherein a mechanical clamp for holding the positioning guide is engaged with said positioning guide to fix a position of said disc-shaped substrate.
(8) The method according to any one of (3), (4) and (6), wherein said positioning member is a mechanical clamp engaged with an inside of said circular recess.
(9) The method according to any one of (1) to (8), wherein the disc-shaped substrate has no central hole and has a continuous surface from its periphery to the center thereof before completing the sputtering step.
(10) The method according to any one of (2), (3), (4), (6), (8), wherein, in the sputtering step, said carrier includes a holder surface with flat and watertight structure, said holder surface makes contact with an outside region of said circular recess on the non-sputtering surface of the disc-shaped substrate to hold the disc-shaped substrate, said holder surface being cooled with a gas refrigerant from a backside thereof.
(11) A disc-shaped substrate for a disc-shaped optical recording medium, on a surface of which at least one-film is formed by sputtering, the substrate comprising: a sputtering surface being one surface of said disc-shaped substrate, said sputtering surface being a continuous surface from a periphery to a vicinity of the center thereof without a central hole for optical medium chucking; a non-sputtering surface being the other surface of said disc-shaped substrate, having a circular recess formed at a center of said non-sputtering surface.
(12) The disc-shaped substrate according to (10), wherein a removal region is formed inside said circular recess at the center of the disc-shaped substrate to form a central hole for optical medium chucking.